1. Field of the Invention
The invention relates to a method of determining the spatial and/or spectral distribution of the nuclear magnetization in separate regions within an object.
For MR examinations (MR=Magnetic Resonance) it is often desirable to examine spatially separate regions of an object:
a) For example, for in-vivo spectroscopy it is important to acquire reference spectra from non-pathological regions for the evaluation of the spectral nuclear magnetization distribution in a diagnostically relevant region. PA1 b) In the MR imaging processes often only given regions of the overall image are of diagnostic interest, so that it is not necessary to image the entire examination zone. The possibility to image only given regions can then be advantageous if the examination zone contains highly vascularized regions which are not of diagnostic interest and could cause strong flow artefacts. In that case it is desirable to excite and to image only the other regions. PA1 c) Furthermore, it is often important to generate separate fat or water images in different regions, for example in two different planes. The fat or water images are then formed from different frequency ranges of the MR signals. In this case, therefore, there is a total of four separate regions (two spatially separated and two frequency separated regions) in which the nuclear magnetization distribution is to be determined. PA1 a) Predetermining n linearly independent codes of the phase and/or the amplitude of the nuclear magnetization in the n separate regions. PA1 b) Generating n different multi-dimensional RF pulses which are shaped so that the nuclear magnetization is excited only in the n regions, that is to say always in conformity with one of the n codes. PA1 c) Acquiring of the MR signals appearing subsequent to the excitation of the nuclear magnetization in the regions. PA1 d) Forming linear combinations of the MR signals, or MR data linearly derived therefrom, in order to determine the nuclear magnetization distribution in a respective one of the regions. PA1 a) an RF coil system for generating RF pulses, PA1 b) a gradient coil system for generating temporally variable magnetic gradient fields, PA1 c) a coil system for receiving MR signals, PA1 d) a reconstruction unit for reconstructing the nuclear magnetization distribution from the MR signals received, PA1 e) means for predetermining the phase and/or the code in n separate regions, PA1 f) a control unit for controlling the components in such a manner that the following steps are carried out:
It is already known that the nuclear magnetization can be excited or refocused in arbitrarily shaped, spatially multi-dimensional regions by means of so-called multi-dimensional RF pulses. Multi-dimensional RF pulses are amplitude and/or phase-modulated pulses which, in the presence of temporally varying magnetic gradient fields, excite longitudinal magnetization or refocus transverse magnetization. Such multi-dimensional RF pulses are described in J Magn. Reson. 82, 647-654 (1989). Moreover, the publication Magn. Reson. Med. 15, 287-304 (1990) describes the formation of RF pulses which are spatially and spectrally selective at the same time and enable excitation of each time one of several spatially and frequency-wise separated regions. These pulses are also to be considered as multi-dimensional RF pulses.